RETURN TO THE WORLD SCIENCE HOME PAGE

Feisty mammal dined on dinosaurs 

But scientists have unearthed evidence to overturn that myth, they say – including the fossil of a mammal with a little dinosaur in its stomach.

Two paleontologists from the American Museum of Natural History and their colleagues studied a fossil of a 130-million-year-old opossum-sized mammal called Repenomamus robustus and found the remains of a dinosaur called a psittacosaur in its stomach area. 

This fossil, discovered in China and described in a new paper in the journal Nature, is the first direct evidence that some primitive mammals fed on small vertebrates, including young dinosaurs, said Meng Jin and colleagues, the paper’s authors.

In the same paper, the team also has described the fossil of a much larger and very close relative of the psittacosaur eater, Repenomamos gigantus, which was the size of a small dog—larger than some dinosaurs that lived in the same region of China at this time. 

Together, these two fossil findings show that some mammals of the Mesozoic era (280 to 65 million years ago, just before dinosaurs and numerous other animals became extinction) were carnivores, the scientists said.

These mammals could also grow to be much larger than previously thought, and competed with smaller dinosaurs for food and land, said the researchers,

The dog-sized animal, Repenomamus gigantus, is the largest known mammal ever found with fairly complete fossil remains from the Mesozoic era, the researchers said.  Although it resembled no animal living today, it is somewhat comparable to a Tasmanian devil, a squat, carnivorous marsupial that today lives only on the island of Tasmania, southeast of Australia. 

The fossil was discovered in rock composed of volcanic and former riverbed sediments in northeastern China’s Liaoning Province, where numerous well-preserved non-avian dinosaur and bird fossils have been found in recent years.  Most Mesozoic mammals were the size of today’s mice and rats, weighing a few pounds at most and at a major size disadvantage in the face of predatory dinosaurs. 

However, a full-grown Repenomamus gigantus probably weighed close to 30 pounds and could hold its own against small dinosaurs, said the researchers.

The discovery of Repenomamus gigantus’s size combined with the discovery of the Repenomamus robustus fossil with the psittacosaur in its stomach recasts scientists’ understanding of Mesozoic animals.  It is now clear that an adult Repenomamus could successfully take on a small or juvenile vertebrate, Meng and colleagues said.

“This new evidence of larger size and predatory, carnivorous behavior in early mammals is giving us a drastically new picture of many of the animals that lived in the age of dinosaurs,” Meng said.

A full-scale model of how scientists think Repenomamus gigantus looked in life is being created for a 700-square-foot diorama depicting a prehistoric forest and its inhabitants to be featured in a new exhibition, Dinosaurs:  Ancient Fossils, New Discoveries, opening at the Museum on May 14, 2005.

The last meal of the Repenomamus robustus was discovered in the fossil’s preparation process, which involved painstakingly removing sediment surrounding the fossil.  As the work was under way, preparators made an extremely rare discovery, possibly the first of its kind in a Mesozoic mammal—the animal’s stomach contents. 

They were revealed as a patch of small bones within the adult mammal’s ribcage near the vertebrae where the stomach would be in living mammals.  The bones turned out to be the limbs, fingers, and teeth of a juvenile psittacosaur, a two-legged, parrot-beaked plant-eating dinosaur that was common in the area of China where the fossil mammal was found. 

Adult psittacosaurs, with short, deep heads, grew to be nearly six feet tall and had four-fingered grasping hands.  The baby psittacosaur found in this fossil was only 5 inches long, a third the size of the animal that ate it.  Wear marks on its teeth indicate it was not an embryo.  Some of the psittacosaur’s long bones were still connected to one another in the belly of the mammal, suggesting that Repenomamus swallowed the psittacosaur in chunks. 

Repenomamus robustus had large, pointy incisors, canines, and premolars useful for catching, holding, and ripping prey, further evidence that this group of primitive mammals ate meat as well as plants.  Its robust jawbones and deep pits on nearby bones suggest that large muscles powered this mammal’s jaws.  But its molars were small and blunt.  Along with the evidence of the psittacosaur chunks, the teeth suggest that Repenomamus did not chew its food and that chewing evolved further up the family tree of mammals.  The teeth and jaw muscles also suggest the animal was an aggressive predator, rather than a scavenger.

Prior to the discovery of Repenomamus gigantus, Mesozoic mammals were thought to be primarily nocturnal insect-eaters, limited to tiny meals due to their minuscule size. Scientists hypothesized that these primitive mammals remained small because carnivorous and herbivorous reptiles, already on the evolutionary scene, prevented mammals from competing for the food and territory and evolving to become larger.  Mammals never got a good chance to grow larger, scientists thought, because larger reptiles, including dinosaurs, lived longer, could move faster, and could travel further to find food due to their size advantage. 

The discovery of Repenomamus gigantus calls these notions into question and shows that these large Mesozoic mammals probably roamed just as widely as some of the small dinosaurs with which they competed for food and territory, the researchers said.

When initially naming Repenomamus, scientists combined the words reptile and mammal to refer to the curious shape of these “reptile mammals.”  The resemblance to reptiles can be seen in their large, sharp, pointy teeth and short limbs that stick out from their bodies at an angle.  But their limb joints allowed more freedom of movement than is typical in reptiles.  In fact, their limbs were more mobile than those of mammals such as the waddling platypuses.  This additional freedom of movement, such as is found among marsupials and other more advanced mammals, helps carnivores compete for prey and elude predators. 

* * *